The invention relates to a method for the control of the bandpass width in the filtering of an intermediate frequency signal of an audio receiver of a radio reception apparatus in which a signal parameter of a user signal of the audio receiver which is received at a reception frequency is determined and taken into account. The invention also relates to a corresponding radio reception apparatus with an audio receiver for the reception of a user signal at a reception frequency, an intermediate frequency filter unit for the filtering of an intermediate frequency signal corresponding to the user signal and a filter selection unit for the selection of a bandpass width for the intermediate frequency filter unit of the audio receiver.
In radio reception apparatuses of this kind the user signal, i.e. the wanted signal, is received in an input circuit of the audio receiver by means of an antenna, pre-amplified and converted to an intermediate frequency signal in a heterodyning circuit so that the user signal is for example present as a frequency modulation of an intermediate frequency of 10.7 MHz. The intermediate frequency signal is amplified and bandpass filtered in the intermediate frequency filter unit with a specific bandpass width.
Known radio reception apparatuses have, for the selection of the bandpass width of the intermediate frequency filtering, a filter selector unit in which a suitable signal parameter of the user signal is determined and in which in dependence on this a corresponding bandpass width is set in the intermediate frequency filter unit. If for example a lower reception quality is detected, a narrower bandpass width is selected in order to filter the frequency-wise adjacent disturbance signals out of the user signal.
This filtering with a narrower bandpass width indeed reduces the influence of disturbance signals of this kind and thus improves the selectivity; it however also leads to a worsening of other parameters, such as for example the stereo channel separation. Therefore the filter selection unit causes the use of a greater bandpass width as soon as a higher reception quality is detected. Since the improvement of the reception quality however can be effected by the previously caused use of a narrower bandpass width, this control mechanism can lead to an audible and thus disadvantageous continual change among the various filters available.
An object of the invention is to provide a method for the control of the bandpass width in the intermediate frequency filtering in which a continual change among various bandpass widths is avoided.
This object is satisfied for a method of the initially named kind in that a signal parameter of at least one neighboring signal which is received at a neighboring frequency adjacent to the reception frequency is determined; and in that the bandpass width of the intermediate frequency filtering is selected in dependence at least on the signal parameter which is determined from the user signal, on the signal parameter which is determined from the neighboring signal and on the frequency distance between the reception frequency and the neighboring frequency.
It is thus provided in accordance with the invention to determine and take into account for the calculation and setting of the bandpass width not only a signal parameter which is associated with the audio user signal, but also at least one signal parameter which is associated with a disturbing neighboring signal on a different neighboring frequency which is adjacent to the reception frequency of the user signal.
In the context of this invention a signal parameter of the user signal is to be understood to be a parameter which can be determined from the user signal and which at least indirectly enables a statement on the quality of the signal reception in the audio receiver or on the presence of undesirable disturbing influences. For example the signal level or the signal strength of the user signal, in particular in the form of the electric field strength, or the deviation of the user signal, come under consideration as a signal parameter of this kind. A further possible signal parameter of the user signal can be given by an output signal of an associated reception quality detector, in particular by a so-called ultrasonic-noise (USN) signal, which provides information on the presence of neighboring signals.
In the method in accordance with the invention an unintentional continual change among different filters or filter bandwidths respectively can be more easily avoided, since with the relevant signal parameter of the neighboring signal a selection criterion can in principle be used which is not influenced by a change in the bandpass width for the user signal. As a result of this additional selection criterion, a criterion can also be used as the signal parameter of the user signal which, in comparison with the reception quality, which is taken into account in known methods, is not or is merely insignificantly influenced by a change of the bandpass width of the intermediate frequency filtering.
A signal parameter of the neighboring signal is to be understood in the sense of this invention to mean a parameter which can yield a statement on the strength of the associated neighboring signal and accordingly on the suitability of the neighboring signal for the disturbing influencing of the user signal. For example the signal strength, in particular the electrical signal strength, or the deviation of the neighboring signal can be used as such a parameter of the disturbing signal.
In this manner a direct feedback between the selected bandpass width on the one hand and the determined signal parameters on the other hand is avoided to a great extent, and the known regulation mechanism is replaced by a stable control.
The determination of the respective signal parameter of the user signal and of the neighboring signal can take place particularly effectively when the relevant signal is already transformed to an intermediate frequency signal and as such has been amplified and filtered as well as in particular demodulated again.
In order to select a corresponding bandpass width of the intermediate frequency filtering on the basis of the explained signal parameters of the user signal and the neighboring signal, these signal parameters can be compared relative to one another. It is in particular possible to compute the respective characteristic values with one another, for example to subtract them, to add them and/or to divide them, and in this to provide them with different weighting factors.
Preferably, the bandpass width is in principle selected all the narrower, the higher the ratio of the signal strength of the neighboring signal to the signal strength of the user signal and/or the lower the distance between the reception frequency and the neighboring frequency is.
Furthermore, it is preferred when the neighboring signal is determined by means of a separate additional receiver, the reception frequency of which, i.e. the neighboring frequency, can be set independently of the reception frequency which is set in the audio receiver. In this way the additional receiver can continually monitor different neighboring frequencies so that the current setting of the bandpass width of the intermediate frequency filtering can be continually monitored and corrected when appropriate, without it being necessary for the audio receiver to interrupt its reception at the reception frequency for this.
If the signal parameter of the neighboring signal can be determined in a correspondingly short time, it is however in principle also possible to receive the neighboring signals which are required for the selection of the bandpass width by means of the audio receiver, in that the latter continually changes, for short times and thus not perceivably by the listener, from the reception frequency to the relevant neighboring frequency and back.
If the neighboring signal is determined by means of an additional receiver, it is preferred when the latter is designed as a data receiver so that digitally coded radio data, in particular RDS signals, of all available radio stations can be evaluated by means of the additional receiver. These radio data can, for the reception frequency which is set in the audio receiver, for alternative reception frequencies of the radio program which is set in the audio receiver or for other radio programs, be determined, evaluated and/or held in readiness for the case of a subsequent manual change of the radio program.
In particular it is possible to take into account the bit error rate of an RDS signal which has been received by the additional receiver or by the audio receiver as an additional criterion for the selection of an intermediate frequency bandpass width.
It is of course possible to monitor many different neighboring frequencies. The respective determined signal parameter of a neighboring signal can for example be taken into account in that after each setting of a new neighboring frequency it is examined whether the disturbing influence of the corresponding neighboring signal requires a change of the currently set bandpass width of the intermediate frequency filtering. For this a current value of the relevant signal parameter of the user signal can be determined. In addition, between such monitorings of different neighboring frequencies, it can also be examined anew on the basis of which the last change of the bandpass width was carried whether those signal parameters of the user signal and the neighboring signal out still have the same values. In these examinations predetermined tolerances can be taken into account.
The monitoring of different neighboring signals or neighboring frequencies respectively is preferably limited to a restricted frequency band in the neighborhood of the user signal. For example in ultra short wave (frequency modulation) reception this monitoring can take place in frequency steps of 100 kHz, namely in a frequency neighborhood of xc2x1300 kHz with respect to the reception frequency of the user signal.
In order to set a bandpass width which has been selected for an intermediate frequency filtering, one of a plurality of available filters of different bandpass width can be activated, or the filter properties of a digital filter can be varied quasi continuously in small steps and within predetermined extreme limits. It is possible to filter the intermediate frequency signal in dependence on the result of the explained monitoring asymmetrically, i.e. with different bandwidths in the positive and negative frequency neighborhood of the user signal.
It is a further object of the invention to create a radio reception apparatus in which a continual change among different bandpass widths in the intermediate frequency filter unit caused by feedback effects is avoided.
This object is satisfied for a radio reception apparatus of the initially named kind in that an additional receiver which is substantially independent of the audio receiver is provided for the reception of a neighboring signal on a neighboring frequency which is adjacent to the reception frequency of the audio receiver; in that a signal parameter of the neighboring signal of the additional receiver and a signal parameter of the user signal of the audio receiver can be determined by the filter selection unit; in that the filter selection unit has a signal processing unit for the computation at least of the signal parameter of the user signal, of the signal parameter of the neighboring signal and of the frequency distance between the reception frequency and the neighboring frequency with one another; and in that the bandpass width of the intermediate frequency filter unit of the audio receiver can be selected in dependence on an output signal of the signal processing unit.
Through the provision of a special additional receiver and a filter selection unit for the determination and corresponding processing of the signal parameters of the user signal and the neighboring signal, the advantages which have already been explained in connection with the method in accordance with the invention can be achieved.
The radio reception apparatus can be designed in accordance with the explained embodiments of the method in accordance with the invention. In particular the filter selection unit can have a frequency determination device for the determination of the distance between the reception frequency and the neighboring frequency. The signal processing unit can be formed by a microprocessor.